Investors are required to pay taxes on their cryptocurrency investments if they sold their digital-coin holdings at a higher price than they bought them. But since many cryptocurrency exchanges don't send customers tax documents, many may not even know they have to pay taxes on their coins — or how much.

Perry Woodin, the founder of Node40, a blockchain accounting company recently acquired by HashChain, told Business Insider this was a problem affecting many investors.

"Millions of people have a tax liability they don't realize or know how to get the numbers for," Woodin said.

"Around December 2017, I got caught up in the altcoins frenzy and sold most of my bitcoins (about $120k worth) to buy a bunch of different coins. I didn't know this back then but it looks like I owe income taxes on those trades, which adds up to about $50k if I add up state (California) and federal."

The problem is that the coins the investor swapped their bitcoin for are now worth only about $30,000. And this person says they have only an additional $5,000 in savings.

"I feel like I might have accidentally ruined my life because I didn't know about the taxes," the investor concluded.

Woodin told Business Insider he knew a couple who faced a similar situation.

"In this case, they bought Dash real cheap at $7, and then it went to $1,600," Woodin said. "They cashed out to do $100,000 worth of home renovations and didn't have enough to pay the tax."

At first glance, it might seem easy to chalk up these anecdotes as irresponsible behavior. But there are some good reasons people are confused.

For the most part, cryptocurrency exchanges do not send customers tax forms, such as a 1099-DIV, which informs investors how much they made from selling investments during the year. The exchange Coinbase does send this tax information, but only if you have more than $20,000 invested with the company.

Without a 1099-DIV, it's up to the investor to figure out how much they made from selling cryptocurrencies during the year and how much they'll owe in taxes.

The tax rate depends on how long the cryptocurrency was held. If it was less than one year, the investment profit is taxed just like a salary. If it was longer than one year, capital gains tax rates apply — they're typically lower but can be as high as 20%.

So what are you to do if you are a crypto investor in a tax-related bind? According to Gwen Moore, a partner at the tax consultancy Johnson Moore, the bottom line is to reach out to the IRS.

"If they have a huge tax bill they can't afford, then they can get into a payment plan," she said. "There are certain programs people can get in to settle with the IRS for taxes if they can't pay."

"People can take action to find out what their cost basis is, what the potential gain or loss would be," Moore said.

If you filed without mentioning your cryptocurrency investments, it's OK, Moore says, as long as you notify the IRS and explain your situation.

"They have a lot of discretion to distinguish between people who made a mistake and someone who is evading taxes," she said. "Obviously, there is a difference between someone who comes forward and explains their mistake, and someone who looks like they heard about the crackdown and moved their crypto somewhere else."